A vehicular radar device that detects an object ahead of a vehicle is proposed in JP-A-2002-40139. The radar device emits light waves or millimeter waves forward and detects an object based on reflected waves. This kind of radar device is used in a warning system that provides a warning when the vehicle becomes close to an object in front, such as a vehicle in front. It is also used in a speed control system that controls a vehicle speed to maintain a predetermined distance to a vehicle in front.
In the radar device, a laser diode emits laser beams as outgoing waves. The laser beams are reflected with a rotating polygon mirror. Multiple laser beams are emitted in a predetermined range with predetermined horizontal and vertical limits. The laser beams reflected by an object are received by the radar device through a light sensitive lens. The received reflected beams are guided to a light sensitive element. The light sensitive element outputs an electrical signal indicating light levels. The radar device determines a distance to the object based on the time when the electrical signal reaches a predetermined voltage after the laser beam is emitted. It also determines horizontal and vertical positions of the object based on an emission angle of the laser beam.
A protection cover, such as a glass plate or resin plate, are attached to laser beam emitting and receiving surfaces to protect light emitting element and the light sensitive element. The laser beams may be scattered when the protection cover has a dirty spot, and some of scattered laser beams may be received by the light sensitive element. As a result, performance of the radar device decreases.
Moreover, the received scattered laser beams may become noise components in object detection. Thus, object detection performance greatly decreases when the protection cover has a dirty spot.